1. Field of the Invention
The present invention relates to a multiplexed signal transmitter which frequency-division multiplexes a television signal, compatible with the existing television system, with a television signal of a high-definition system, and transmits the multiplexed signal, and a multiplexed signal receiver which receives the multiplexed signal.
2. Description of the Related Art
Recently, research for and developments of a high-definition image have become prominent in the television broadcasting. In this high-definition system, a television signal of the existing system as a main signal is frequency-division multiplexed or frequency interleaved multiplexed with a television signal of a high-definition system as a helper signal in order to realize compatibility with existing systems. The helper signal typically includes a high-frequency component of a luminance signal or a signal or information corresponding to the side portion of a wide-aspect system. There are two frequency-division multiplexing systems of the above type: one is disclosed in T. Hukinuke et al., "Extended Definition TV Fully Compatible with Existing Standards" IEEE TRANSACTIONS COMMUNICATIONS, VOL. NO. 8, AUG. 1984 (hereinafter referred to as Publication 1) and the other is disclosed in M.A. Isnardi et al. "Encoding for Compatibility and Recoverability in the ACTV System" IEEE TRANSACTIONS BROADCASTING VOL. BC-33 NO. 4, DEC. 1987" (hereinafter referred to as Publication b 2).
The helper signal for providing high definition of the main signal does not contribute to image reproduction in the existing television receiver, and it is treated as a noise signal to the main signal. This would raise a problem of interference of the helper signal with the main signal. The helper signal is effectively used only in a television receiver of the high-definition system.
To eliminate the influence requires reduction in multiplexed level of the helper signal.
Reducing the multiplexed level of the helper signal, however, reduces the signal-to-noise (S/N) ratio so that the helper signal cannot be accurately reproduced in the high-definition television receiver.
Since the helper signal for the high-definition system is a high-frequency signal, the averaged power of the helper signal is considered comparatively lower than the averaged power of the main signal in view of the general property of the television signal. In this case, therefore, the interference of the helper signal with the main signal appears insignificant. However, this actually raises a problem of increasing the peak value of the main signal at the edge portion of an image. Because the edge portion of an image corresponds to a high-frequency signal, further multiplexing this signal with a helper signal (high-frequency signal) would cause the multiplexed signal to exceed a transmittable level. As a result, a reproduction signal of the edge portion of the image on the receiver side becomes inaccurate.
In order to solve this shortcoming, the system disclosed in the aforementioned Publication (2) employs a method of transmitting a helper signal in a non-linearly compressed format on the sender side and non-linearly expanding this signal for reproduction on the receiver side.
This method can reduce the interference of the helper signal with the edge portion of the main signal without reducing the S/N ratio of the helper signal.
This method, however, would raise the following two problems.
(1) First, the reproduced output of the helper signal has waveform distortion. In a non-linear compressing process, a harmonic component is generated. In transmitting a helper signal frequency-division multiplexed with a main signal, on the other hand, the transmission band of the helper signal is generally set to the same band as the spectrum band of the helper signal prior to compression in order to improve the efficiency of utilizing the transmission band. In this case, therefore, the harmonic component generated in the non-linear compressing process on the sender side would not be transmitted to the receiver side. Consequently, the helper signal acquired by the non-linear expanding process on the receiver side would have waveform distortion.
(2) The second problem is that such an arrangement causes visually unnatural deterioration of the quality of an image. In executing digital processing of the helper signal to be transmitted as an analog signal on the receiver side, an 8-bit circuit is often used as an analog-to-digital (A/D) converter. This arrangement is used because a 10-bit circuit significantly raises the cost for the overall circuit. Although the 8-bit circuit has a resolution of 256 gradations, it cannot ensure 10 to 20 gradations for a helper signal because the peak value of the helper signal should be compressed to 1/10 or less in order to eliminate the interference of the helper signal with the main signal. Such reduction in resolution not only increases quantized noise but also provides a visually unnatural image. In the above example, particularly, the reduced resolution would be further augmented in the non-linear expanding process carried out on the receiver side, thus making the unnaturalness of the reproduced image more prominent.
As described above, in the prior art apparatuses which frequency-division multiplex a helper signal for high definition, having a correlation with a main signal, with this main signal and transmit the multiplexed signal, although it is possible to eliminate the influence of the helper signal with the edge portion of the main signal, the helper signal cannot be accurately reproduced. The mentioned correlation is such that when the level of the main signal is high, the level of the helper signal is high, and when the level of the former signal is low, the level of the latter signal is also low.